Agricultural fields are often sprayed with various spraying solutions, such as herbicides, insecticides, fertilizers, etc. Sprayers for this purpose are required to have a wide span booms furnished with plumbing and multiple spray nozzles, for the liquids being sprayed, at defined intervals across the boom's span: While contemporary span widths may extend as wide as 120 through to 162 feet, with even wider spans predicted in the future. However, when not being used for spraying, these wide-span booms are required to fold away to a stowed position, typically along either side of the vehicle, to permit the vehicle to navigate field entrance gates and traverse tracks, roads or highways, without exceeding either practical or legal width limits.
The vehicles which employ these foldable wide span booms may take the form of farm tractors, trailers or specialist vehicles fitted with chemical tanks or reservoirs to carry the liquids being sprayed, and the spray booms themselves may be fitted typically at either the front or rear of such vehicles on each side of the vehicle on a “center-rack” which also supports plumbing and spray nozzles across the short distance that spans the width of the vehicle itself. Typically, but not universally, this center-rack, along with the booms attached to it, can be elevated or lowered on a four-bar linkage and actuator(s) or other suitable means, to adjust and set the boom/nozzles spray height to achieve to most effective crop spray coverage. Typically, at their attachment to the outer edges of the center-rack, the booms are arranged to pivot about essentially vertical axes, so that they may be rotated in angular displacement through 90° or so from the spraying position which is normal to the vehicle's longitudinal axis, to the folded, stowed position essentially parallel the vehicle's longitudinal axis when viewed in plan. Because of the great length of the booms, it is customary for the booms to be further folded via hinge points located at around the mid-semi-span of each boom. In some cases, where very high span booms are used, further folding hinge points may be used to shorten the folded boom length. In other cases, the folded boom length may be shortened by having the outer semi span of each boom retract telescopically in to the inner boom semi-span. In yet others, a combination of telescopic and folding boom segments may be employed. In most cases, the folding and unfolding action of the booms is conducted by means of actuators, more commonly, hydraulically operated. Conveniently, when the full span of the booms may not be required (because of width limitations of parts of the field being sprayed, for example) spray booms are commonly designed to be operated in the part-folded position, with the outboard sections folded alongside the inboard sections. Thus a boom system might be referred to as a 132-60, or other similar designation, implying in this case a full span of 132 feet, and a semi-folded span of 60 feet.
The loads imposed on the booms during operation have a significant effect on their structural design. It is the mass of the booms' structure itself along with its supported loads including pipework, plumbing, spray nozzles, valves, filters, hydraulic cylinders and sundry masses such as touch-down wheels that is responsible for generating the greater part of its structural loading.
Since the mass of the boom assemblies, as described above, acted on by gravity and by inertial accelerations in the vertical direction, about the roll axis and by inertial and some gravitational components in the longitudinal direction as well as about the yaw axis; maximizing the specific strength of the booms by minimizing their mass relative to their structural strength is an extremely important, if not critical aspect of high-span spray boom design. It is therefore highly advantageous to design high-span spray booms using high-strength lightweight materials and to incorporate specific design features that simplify and aid manufacture, while keeping costs to a minimum.
Of the destructive loads able to be imposed upon the deployed booms by the movement of the vehicle as it traverses the undulating surface of the farm land being sprayed, the vehicle's movement about the roll and yaw axes are potentially the greatest. This is because the booms effective moment of inertia about these axes can be defined as the sum of an infinite number of discrete mass segments each of whose moment is the product of the segment mass and the square of its distance from the roll axis: Therefore because of the large boom span and the distance-squared function, the polar moment of inertia of the deployed booms is truly massive, notwithstanding that the booms' outboard sections can be comparatively light. Accordingly, if the vehicle moves in angular displacement about its roll axis due to continuously varying relative vertical displacements of the wheels at either wheel track, then enormous potentially destructive forces may be generated at the booms' attachments to the center rack, unless some mitigating design features are incorporated to prevent or reduce such forces.
One way that this is currently done is to arrange to allow angular displacements to readily occur between each inner boom and the center-rack at a lower, acceptable level of force, by having a lower longitudinal pivot between the boom and center-rack. Each boom is then maintained in an essentially horizontal position by a hydraulic cylinder that attaches to an upper inboard boom attachment point at one end and to an upper center rack attachment at the other end. By incorporating relatively small pressurized gas accumulators in the positively pressurized hydraulic supply lines to these cylinders, a level of compliance (springing) in angular roll displacement can be achieved between each of the booms and the center-rack/vehicle. Further, by arranging for a control system to lengthen or shorten the each upper boom attachment cylinder, each boom can be controlled independently in angular displacement about the vehicle's roll axis; and this can be used, in conjunction with ground-height sensors mounted, typically, at intervals across the span of the booms, as part of a system to control and maintain to booms essentially parallel to the ground during operation. It also permits control recognition of sudden vehicular roll movements and allows active correction of the boom position relative to the vehicle's roll position when such spurious movements occur. While this has proven to be effective, at least to some extent, the active control response is often considered to be too slow to be fully effective in mitigating the movements and forces caused by sudden roll excursions of the vehicle. Consequently, over the more undulating surfaces, the vehicle speed may have to be reduced to unacceptably slow levels to allow time for the corrective response to take place, or loss of effective control of the outboard boom section heights may take place, giving rise to unnecessarily high boom forces in roll as well as defective spray application and may even cause the boom tips to impact the ground.
Again, according to contemporary practice, the foregoing active roll correction system is sometimes further improved by linking the hydraulic lines feeding two upper boom hydraulic cylinders on either side of the vehicle together via pressure relief valves: In the event that the roll forces imposed on the cylinders gives rise to a hydraulic pressure differential between the cylinders that exceeds a given pre-set level, the relief valves then open and hydraulic fluid is transferred automatically between the two cylinders, allowing the vehicle to effectively roll relative to the pair of booms en-masse without having to react further forces. This may well be an improvement, but it is not a full solution since typically outer boom height control is still rendered somewhat ineffective in practice.
An alternative way that the potentially destructive loads caused by the vehicle's roll excursions from reacting the polar moment of the booms is currently addressed, is to mount the center-rack, to which the booms are attached, on a longitudinally aligned pivot at the center-rack's primary attachment to the vehicle. A torsionally resilient connection may be used at this point and this may take the form of torsionally acting spring elements or other means to help keep the boom in general horizontal alignment, relative to the vehicle, without the vehicle's short term roll movements significantly deflecting the booms in roll, or generating excessively high reaction forces in the booms at their attachment to the center-rack. A further variation on this theme is for the center-rack to be supported by a linkage that results in an effective virtual longitudinal pivot point whose virtual pivotal axis is above the center of gravity of the combined booms and center rack, such that the booms benefit by the pendular stability so generated, at least when travelling on fairly level terrain. The upper boom attachment hydraulic cylinders, or a single hydraulic cylinder and linkage serving to replace them, then acts to change the angular displacement of the pair of booms in roll relative to each other, rather than relative to the vehicle.
A second, independent, control action may then be employed to control the overall position of the linked pair of booms in roll, relative to the ground reference, given by the previously mentioned boom height sensors. Thus, by controlling these two sets of boom roll position criteria, the booms may not only have the roll forces, otherwise imposed on them by the vehicle movement over undulating or rough ground surfaces, reduced or effectively eliminated, but a comprehensive boom height control system can effectively permit the booms to be maintained at an essentially fixed mean-height above the ground; and also that this mean height can be maintained even when traversing the rounded crest of a hill or ridge, or along a gully by virtue of being able to control the roll position of the booms relative to each other at the same time. Thus, the spray booms are better able to follow, at an essentially constant mean height above the ground, any gently varying contours of the ground that occurs across the span of the booms during operation.
Again, according to contemporary practice, there are two recognized methods by which the active control force can be applied to control the mean angular position of the linked pair of booms relative to the ground: one is to react the controlling actuator in roll against the vehicle, while typically incorporating an interposed low spring-rate compliant element, such that the reaction force is rendered at least somewhat independent of the relative angular roll axis position of the vehicle: While the other is to change the lateral position of the combined booms' center of gravity relative to the center-rack's roll pivot support, such that gravitational reaction is used. This latter concept has the advantage of deriving the boom roll control forces entirely independently of the vehicle's instantaneous roll position. This can be achieved, for example, by displacing weights slidably attached to the booms, laterally in order to apply corrective roll forces. One example of such an arrangement is disclosed in WO2012146255, “Active Damping System for a Spray Boom”, Maagaard Jorgen, 2012.
On a practical note, one boom design feature that has become almost universally adopted by current wide-span spray boom designs is the “breakaway”. This is typically a vertical hinge pivot system applied such that the last outboard 12 to 15 feet or so of the boom, up to the boom tip itself, can pivot back to alleviate damage if the outermost extremities of the boom accidently contact an obstacle, or contact the ground. There are a number of ways that this is achieved in practice, one most common one being of the double pivot “saloon-door” hinge type, where the breakaway section is centered in the fully extended position by pin inclination and gravity or by spring force, or both, so that upon contact with an object, the breakaway section fold back to avoid damage, and re-centers automatically when the object or ground contact has passed.
Another practical adaption often used on wide-span agricultural spay booms are so called “touch-down” wheels. These wheels are attached on legs, one on each boom semi-span below and slightly forward of the booms to avoid interference with the spray pattern, fairly well outboard along the boom span. Their purpose is to prevent the booms from encroaching too close to, or touching the ground in the event of the control system failing to adequately maintain the correct height position of the boom. While such touch-down wheels may prevent obvious damage to the booms in the event the height control system failure, their inclusion might be considered as indication of the inadequacies of current spray boom/control system design and control methodologies and the need to address them.
Structural design is of vital importance to both the affordability and durability of wide-span spray booms. In this respect it is not only the absolute structural strength of the booms that is relevant, but also, and perhaps more critically, the fatigue strength, which on metal boom structures, particularly welded metal boom structures, usually defines the boom's usable life. In this respect the amplitude of the cyclic fatigue loadings applied to the boom, either as imposed loads (from bumps in the terrain reacting the inertia of the boom structure, for example) or as resonance generated loads (from structural modal resonance response) are of great importance. This is because the characteristic fatigue S-N curves (cyclic Strain amplitude verses Number of strain reversals to failure) follows a logarithmic curve with a slope of approximately three, so effectively represents a number of cycles to failure that varies inversely as the cube of the cyclic strain. To put this into perspective, if by the severity of operation, the magnitude cyclic loading forces on a given boom structure were to be doubled, then its fatigue life would be expected to fail prematurely at around just one eighth of its original value. While, on the other hand, if by design, the cyclic loading were to be halved, then the same boom would be expect to benefit by an eightfold increase its life.
The alleviation of fatigue loads by adequate compliant suspension in heave (vertical accelerations imposed when traversing undulating or bumpy ground), in roll (which has been addressed in the foregoing paragraphs), in longitudinal acceleration (acting inertially to flex the booms backwards and forwards on accelerating and braking or climbing or descending gradients) and in yaw (accelerations imposed about the yaw axis by steering the vehicle), is commonly practiced in current designs. In some cases, semi-active control of the longitudinal and yaw accelerations is also currently practiced, while automatically self-leveling the booms relative to the sensed ground position at a pre-set spray height, combined with compliant boom suspension, effectively results in semi-active vertical boom suspension, there still remain some serious deficiencies in structural and fatigue boom design capabilities.
Primarily these relate to the propensity to structural resonance in the (necessarily very flexible) booms excited by vertical and/or longitudinal accelerations in the supporting vehicle due to its operation over rough or undulating ground, even when the best methods of trying to isolate the booms from such critical vibration frequencies have been employed. Such resonant vibrations, magnified by an exciting frequency, can rapidly fail or fatigue the boom's structure prematurely. Further, designing to avoid the critical frequencies generated by the vehicle is largely thwarted by the potentially wide range of frequencies able to be generated due to mass of the vehicle changing on its suspension and tires, as its liquid cargo is discharged during the spraying operation. This is a significant weakness in contemporary high-spans pray booms, and one which will only become worse as economic necessity drives future spray boom spans wider.
The optimal structural design of spray booms typically results in a triangulated braced truss-structure for several reasons. Firstly, the truss type structure is one of the strongest, lightest and most rigid configurations, and secondly, when in the folded position along both sides of the vehicle, the open lattice frame of the truss structure allows the driver a fairly high level of visibility through the structure itself, so enabling safer operation, particularly on roads and highways. The open lattice structure also permits ready access to any plumbing, hydraulics, electrical and communication lines, sensors etc. for maintenance or modular adaptability.
From the foregoing it can be seen that there are a large number of relevant factors that need to be addressed in the optimal design of wide-span spray agricultural booms, and that contemporary designs are deficient in a number of respects.
It is desirable that wide-span spray booms be designed using lightweight high-strength materials, so that the booms' span can be maximized while the structural loads, resulting largely from the boom structural mass, can be kept with the limits defined by the operational life requirements.
It is desirable that, in order to maximize agricultural sprayer utility in terms of area sprayed in unit time that both the boom-span and vehicle speed be maximized; notwithstanding that both these parameters significantly increase the propensity of the boom structure to flexure and resonance.
It is desirable for the boom system to be able to accurately maintain the optimal, near constant, spray height above the ground, and to follow the smooth contours and undulations in the ground surface profile in span at the highest practical vehicle spraying speed.
It is desirable that not merely the limit-load strength of the boom structure, but the fatigue strength of the structure, be primary criteria for spray boom design.
It is desirable that the problem of boom structural resonance, particularly in the vertical and longitudinal vehicle axis directions, be eliminated or reduced to acceptable levels, particularly at the resonant Eigen-frequencies.
It is desirable that the spray boom structure be of the truss or lattice type, so that the vehicle driver's visibility through the structure is not significantly impeded when the booms are in the folded position along both sides of the vehicle.
It is desirable that methodologies to mitigate otherwise excessive loads from being imposed on the booms' structure and attachments to the vehicle due to the vehicle's angular displacements in roll over uneven ground being reacted against the deployed booms' extremely high polar moments of inertia in roll
It is desirable that methodologies to mitigate otherwise excessive loads from being imposed on the booms' structure by the vehicle's movement over rough or uneven ground, in the vertical or longitudinal axis directions due to the booms' inertia reacting the vehicle's vertical, longitudinal and yaw displacements.
It is desirable that, in the design of the booms combined with their attachments to their supporting center-rack, along with the center-rack's attachment to the vehicle, that provisions be made to support the use of advanced active boom control methodologies: These are methodologies that enable the booms to follow the varying contours of the ground with a high level of accuracy, without interference from spurious short term vehicle displacements, and with a response time consistent with these objectives.
The present invention serves to overcome these deficiencies.